The present invention relates to amplifiers, and more particularly to an improved polyphase pulse duration modulation (PDM) amplifier.
Pulse duration modulation is sometimes used in power amplifiers to achieve efficiency of operation. In these amplifiers an input signal is used to pulse duration modulate a carrier signal, and the resulting PDM signal is amplified. The amplified PDM signal is then filtered to recover a demodulated signal corresponding to an amplified input signal. The patent to Swanson, U.S. Pat. No. 4,164,714, discloses a polyphase PDM amplifier wherein the amplified signal is formed by combining the outputs of plural parallel-connected PDM amplifiers. All of the amplifiers respond to a common input signal, but have differently phased PDM carrier signals. Polyphase PDM amplifiers of this type enjoy various advantages over earlier, single phase PDM amplifiers. The amplifier elements used in polyphase PDM amplifiers operate at relatively low switching frequencies and power levels, permitting semiconductor devices to be used as the active amplifier elements instead of vacuum tube devices.
The filters used in PDM amplifiers to demodulate the PDM signal should remove substantially all of the frequency component artifacts of the PDM amplification process. This can present a formidable problem, however, where the frequency of the spurious component is near the frequency range of the amplified signal. Very selective filters can provide the desired performance, but may be rather complex and expensive. Less expensive filters may not provide the desired degree of attenuation of the spurious components.